The present invention relates to producing a color which matches an original color. For example, the invention may be used in printing a color which accurately matches a color displayed on a cathode-ray tube (CRT) display.
In order to produce a printed color corresponding to a CRT color, for example, it is necessary to convert the signals which generate the CRT color into signals controlling the printing process. The CRT color signals conventionally include three color coordinates for each distinct colored feature. Each feature could be a picture element, or pixel, or could be any other geometric shape, according to conventional image generating techniques applicable to CRT displays, to printing processes and to other color imaging techniques. The three coordinates may correspond, for example, to the primary colors produced by light emitting elements within each pixel. But these coordinates do not correspond to the toner colors used in printing, and therefore cannot be used directly to control the printing process.
One system of CRT color uses RGB coordinates corresponding to intensities of red, green and blue (RGB), the colors detected by cone cells on the retina of the human eye. These coordinates cause the light emitting elements to generate a CRT color additively, with light of the primary red, green and blue colors mixing to produce other colors. The mixture of all three primaries yields white at maximal intensity and black at minimal intensity. In addition, each pair of the RGB primaries mix to produce a color which can also serve as a primary, with red and green producing yellow, red and blue producing magenta, and blue and green producing cyan. The range of colors, or the color space, obtainable from the RGB coordinates may thus be graphically represented as a cube, with two diagonally opposite corners corresponding to black and white, and the intermediate corners corresponding to the primary colors, as shown in FIG. 1. FIG. 1 also shows the corners corresponding to the cyan, magenta and yellow (CMY) primary colors nearer to the white corner than the RGB primaries are, because each of the CMY primaries is produced by adding the light from two of the RGB primaries, and therefore is more intense and closer to white than the RGB primaries. This illustrates the generation of a color additively.
In contrast to a CRT display, a color printer generates an image subtractively, by applying toners to a white medium (or a transparent medium if back-lit), each toner absorbing some light frequencies and reflecting or transmitting others to produce its characteristic color. The toners commonly correspond to the CMY primary colors, with application of no toner producing white and all toners at maximal intensity producing black. Alternatively, black may be produced with a separate toner. The toners may be mixed by superimposing them or by mixing them in a pattern such as a dot pattern. Superimposing a pair of the CMY primaries produces an RGB primary which can also be used in printing. In general, the mixing of subtractive primaries produces colors within a space similar to the RGB color space of FIG. 1, but a mixed color produced by combining quantities of the primaries will be nearer the black corner of the color space than the same quantity of any of the primaries alone because each added primary will subtract more of the light frequencies than the white medium would.
One technique for converting CRT color signals into printer color signals is to directly convert each CRT color coordinate into coordinates for the pair of CMY primaries which mix to produce that coordinate. This does not accurately reproduce the CRT colors, however, because of spectral differences and because additive and subtractive images differ, as explained below.
The differences between additive and subtractive primaries can be understood by comparing spectral shapes. The intensity spectrum of each additive primary preferably has a narrow peak at the appropriate wavelength and an intensity of zero for other wavelengths. In contrast, the reflectance spectrum of each subtractive CMY primary, as shown for typical toners in FIGS. 2A-2C, spreads across a relatively broad range of wavelengths, and typically includes some light from nearly all wavelengths, with maximal reflectance of the two RGB primaries which additively produce that CMY primary. Therefore, even if mixing of two CMY primaries produces a subtractive RGB primary, the reflectance spectrum of that RGB primary will be much different than the intensity spectrum of a corresponding additive RGB primary. The subtractive primary looks different than the corresponding additive primary.
FIGS. 2A-2C also illustrate why additive and subtractive images differ. The values shown for reflectance are each equal to (1-absorption), so that many wavelengths are almost entirely absorbed, especially by the cyan and magenta toners. Therefore, mixing toners in a pattern will result in further absorption, producing a darker color than the primary toners which are mixed, as noted above. On the other hand, the additive CRT primaries, when mixed, produce a color of greater intensity than the primaries themselves, and the CRT provides colors of higher intensity than can be obtained by ordinary printing, in which the maximum possible intensity is the white of the paper surface. The intensity differences between additive and subtractive colors are another reason the CRT color coordinates cannot be directly converted to CMY coordinates.
A second technique for converting original color signals into printer color signals is described briefly in U.S. Pat. No. 4,446,470. This technique uses density measurements of the CMY primary toners to obtain a compensation matrix. For each original coordinate, the matrix provides a compensation constant to be multiplied with the coordinate to obtain an adjusted amount of each CMY primary toner to be printed. The density measurements are obtained from a solid print of the pure primary toner. Therefore, the measurements may not accurately reflect the manner in which the toners are applied by the printer. For example, the green produced by combining yellow and cyan may be bluish-green because the printer applies less yellow than expected in a pattern. The matrix does not take such variations into account.
A variation of this second technique, described in Starkweather, G. K., "A Color-Correction Scheme for Color Electronic Printers," Color Research and Application, Vol. 11, Supplement (1986), pp. S67-S72, is to use a large number (e.g. 512) of compensation matrices, each matrix being tuned to a small portion of the color gamut. This is still not satisfactory for high saturation colors, however, and requires lengthy, complex computation to generate the matrices. Furthermore, if a color shift occurs due to a change in toner spectrum, toner sequence, dot overlap, flare or paper characteristics, all of the matrices must be recalculated.
A third technique for converting CRT color signals into printer color signals is described in Kenney, J., "Careful Color Matching Makes Hardcopy Output Conform to CRT Display", Computer Technology Review, Fall 1985, pp. 167-175. This technique uses the CIE (Commission Internationale de l'Eclairage) system which assigns coordinates to colors according to their appearance under a standard illumination as viewed by a standard observer. Three coordinates are assigned to each other, but these coordinates are not other colors making up that color; rather, the coordinates represent a summation of the color contributions of all wavelengths within the spectrum of a color sample. These values are mathematically useful in relating measurements of wavelength and intensity to perceived colors. A color on a CRT display is matched by identifying its RGB components or coordinates, determining the corresponding CIE values, and defining the closest CMY mixture using a set of color lookup tables. Non-linear factors such as overlapping ink dots and absorption characteristics of the paper are then corrected. If changes are made in the toners or other printing parameters used in this technique, it would be necessary to develop a new set of color lookup tables.
U.S. Pat. No. 4,522,491 relates to an indirect technique which can be used to reproduce a target color in printed form, but which requires the use of photographic media during an intermediate stage. This technique makes use of coordinates similar to the CIE coordinates to select a period of illumination of the photographic media through each of a number of filters. In addition, the exposure times are adjusted using a correction factor based on color density measurements. This technique is therefore very complicated.
It would be advantageous to have a simple, accurate technique for directly matching printed colors to those on a CRT display which would not require a new set of matrices or tables whenever toner color changes. It would further be advantageous to have a technique which would reliably obtain a printed color closely approximating a CRT display color even when the displayed color is outside the gamut of printable colors.